Well, anger is normal, but I'm not sure what you are dealing with is normal. We all feel anger and yes, it is part of the process, but acting on that anger is another matter. I'd be careful about sending such messages to him because he might save them and attempt to use them to get you into trouble. Can you think of another way to express your anger and get it out, rather than writing or communicating in any way with him? A visit to your doctor might be in order if these irrational thoughts continue.
I'm sorry you are dealing with this. I know many years ago I had such hatred for my ex that I often wished he would just die. I never considered killing him myself, but my hatred ran pretty deep. I knew something wasn't quite right, so I went to my doctor and told her about my extreme anger. She was able to help me with that, and I'm happy to say that after a while, I was able to get on with my own life and quit letting him control me and my thoughts. I still think he's a narcissistic pig, but I don't let him have head space anymore.
I've heard it said that harboring so much anger toward someone is like taking poison and waiting for the other person to die. That makes sense to me.